Ziegler-Natta (ZN) type polyolefin catalysts are well known in the field of producing olefin polymers, like ethylene (co)polymers. Generally the catalysts comprise at least a catalyst component formed from a transition metal compound of Group 4 to 6 of the Periodic Table (IUPAC, Nomenclature of Inorganic Chemistry, 1989), a metal compound of Group 1 to 3 of the Periodic Table (IUPAC), and, optionally, a compound of group 13 of the Periodic Table (IUPAC), and, optionally, an internal organic compound, like an internal electron donor. A ZN catalyst may also comprise further catalyst component(s), such as a cocatalyst and optionally external additives.
A great variety of Ziegler-Natta catalysts have been developed to fulfill the different demands in reaction characteristics and producing poly(alpha-olefin) resins of desired physical and mechanical performance. Typical Ziegler-Natta catalysts contain a magnesium compound, an aluminium compound and a titanium compound supported on a particulate support. The commonly used particulate supports are inorganic oxide type of supports, such as silica, alumina, titania, silica-alumina and silica-titania, typically silica.
The catalyst can be prepared by sequentially contacting the carrier with the above mentioned compounds, for example, as described in EP 688794 and WO 99/51646. Alternatively, it can be prepared by first preparing a solution from the components and then contacting the solution with a carrier, as described in WO 01/55230.
Another group of typical Ziegler-Natta catalysts are based on magnesium dihalide, typically MgCl2, that contain a titanium compound and optionally a Group 13 compound, for example, an aluminium compound. Such catalysts are disclosed, for instance, in EP376936, WO 2005/118655 and EP 810235. The above described ZN-catalysts are claimed to be useful in olefin polymerisation, for example the production of ethylene (co)polymers.
However, even though many catalysts of prior art show satisfactory properties for many applications there has been the need to modify and improve the properties and performance of the catalysts to achieve desired polymer properties and to have catalysts with desired performance in desired polymerisation processes.
Hydrogen and comonomer responses and thus catalyst flexibility as regards to possibilities to control the molecular weight (Mw), polymer molecular weight distribution (MWD) and comonomer content are general indicators of the catalyst performance. Thus, problems relating to these properties indicate performance properties of the catalysts. Further, it's known that if high molecular weight polymer is desired, and the hydrogen amount cannot be reduced anymore, then external additives can be used in the polymerisation. However, in that case polymers are often produced at the expense of the catalyst productivity. There have been several attempts to find solutions by modifying the catalyst. One way to modify the catalyst is to use internal organic compounds. However, even if e.g. the molecular weight of the polymer is improved, often it happens at the cost of some other properties, usually catalyst productivity and comonomer response. Internal organic compounds can be internal electron donors or other compounds affecting the performance of the catalyst, and external additives comprise e.g. external electron donors and/or alkyl halides.
U.S. Pat. No. 5,055,535 discloses a method for controlling the molecular weight distribution (MWD) of polyethylene homopolymers and copolymers using a ZN catalyst comprising an electron donor selected from monoethers (e.g. tetrahydrofuran). The monoether is added to the catalytic component in the presence of the cocatalyst and is further characterised that under no circumstance should the monoethers be brought into contact with the catalytic component without the presence of the cocatalyst in the medium.
WO 2007051607 A1 suggests the possibility of tailoring the properties of a multimodal ethylene polymer by using alkyl ether type internal electron donor, preferably tetrahydrofuran, to modify ZN catalyst component to influence the molecular weight distribution (MWD) of a higher molecular weight (HMW) component.
WO2004055065 discloses solid catalyst component comprising Ti, Mg, halogen and electron donor in specific molar ratios for preparation copolymers of ethylene with α-olefins, where said α-olefins are homogeneously distributed along the polymer chains. The electron donor (ED) is preferably ether, like tetrahydrofuran. Said catalyst component is used in polymerisation together with alkylaluminium compound and optionally with external electron donor. The optional external electron donor is said to be equal to or different from the ED used in catalyst component.
EP0376936 discloses a MgCl2 supported ZN catalyst, where spray-dried MgCl2/alcohol carrier material is treated with a compound of group IA to IIIA (Groups 1, 2 and 13 of the Periodic Table (IUPAC, Nomenclature of Inorganic Chemistry, 1989)) then titanated with a titanium compound, optionally in the presence of internal electron donor. The optional internal donor compound (THF or di-isobutyl phthalate are given in those examples where internal electron was used) is added together with TiCl4 or after adding TiCl4.
However, the activity of the donor modified catalysts of EP0376936 was much lower than the original catalyst without the donor. Moreover, during the donor treatment step, a 10 wt % solution of triethylaluminum and a number of hydrocarbon washings were used, which resulted in a large amount of organic solvent waste.
WO 2014004396 A1 discloses a catalyst component, where bi-heterocyclic compounds are used as internal or external donor. The catalyst is used for propylene polymerisation.
EP 2746306 discloses a supported Ziegler-Natta catalyst component comprising an internal electron donor selected from bi-cyclic ethers. The catalyst of EP 2746306 is prepared by depositing a soluble alkoxy compound of Group 1 to 3 metal, a compound of Group 13 metal, an internal electron donor and a transition metal compound of Group 4 to 6 on a particulate support, or alternatively forming precipitated support material by contacting a soluble magnesium alkoxide compound, an electron donor solution and a solution of aluminium alkyl chloride compound. After precipitation and suitable washing steps the obtained solid support material was treated with a titanium compound to obtain the catalyst component. In this case the molecular weight of the polymer is improved at the cost of catalyst productivity. Moreover, catalyst performance and morphology of precipitated MgCl2 based catalysts are typically sensitive to even small variations in preparation conditions, especially in large scale production.
Although much development work in Ziegler-Natta catalyst preparation has been done, there is still room for improvement. As stated above, some of the methods are particularly sensitive to preparation conditions and/or large amounts of waste material are formed, which are disadvantages in preparing catalyst at a large scale. Modifications of the catalyst synthetic procedure may adversely affect the productivity of the subsequent catalyst so as to not be satisfactory for commercial scale production. Additionally, catalyst morphology can be difficult to control, especially in large scale production. In addition to the needs of catalyst properties and performance, catalyst preparation at commercial-scale should be as simple and robust as possible. Further, the chemicals used in the preparation should be viewed as safe from a health, safety and environment point of view.
Therefore, it would be desired to find a more robust method to prepare the catalyst that allows production at a large scale of a catalyst which is less sensitive to morphology changes with changes in catalyst preparation conditions and chemicals. Further, it is desired that large amounts of waste material during the synthesis can be avoided.
Further, from commercial point of view, the catalyst should show a reproducible composition and performance.
There is also a need to find a catalyst which is able to produce copolymers with wider melt flow rate (MFR) and density windows, such that there is the possibility to produce high molecular weight copolymers with narrow MWD (molecular weight distribution) and high comonomer content combined with low melting temperature.
And finally, the catalyst should have productivity on a level, which makes it useful in commercial polymerisation processes while producing a broad range of molecular weight polymers.
Based on the teachings of prior art, it appears that donor modification might result in the improvement of some properties. However, very often these improvements are made at the cost of catalyst productivity and comonomer response. The MgCl2 based catalysts prepared by precipitation methods are typically sensitive towards changes in preparation conditions.